In a biological organ having a luminal structure such as blood vessels, the trachea and the intestines, when stenosis occurs therein, a cylinder-shaped stent with mesh pattern is used in order to secure patency at a site of pathology by expanding an inner cavity at a narrowed part. These biological organs often have bent or tapered structures locally (i.e. a tubular structure of which sectional diameters of the inner cavity differ locally in an axial direction). Therefore, a stent having higher conformability has been desired which can flexibly adapt to such a complex vessel structure. Furthermore, in recent years, stents have come to also be employed for the treatment of cerebral blood vessels. Among tubular organs in a living body, the cerebral vessel system has a more complex structure. The cerebral vessel system has many bent sites and sites having tapered structures. Therefore, stents with particularly higher conformability have been required therein.
For the purpose of realizing a stent with higher conformability, the two kinds of mechanical flexibilities of a longitudinal axis direction (in a central axis direction) and a radial direction (a direction perpendicular to the longitudinal direction) of the stent are said to be important. Thereamong, the flexibility in a longitudinal axis direction refers to stiffness with respect to bending along a longitudinal axis direction or the ease of bending. The flexibility in a radial direction refers to stiffness with respect to expansion and contraction along a direction perpendicular to a longitudinal axis direction or the ease of expansion and contraction. The mechanical flexibility in a longitudinal axis direction is a property that is necessary for a stent to be flexibly bent along a longitudinal axis direction to allow adapting to a bent site of a tubular organ in a body. The mechanical flexibility in a radial direction is a property that is necessary for making the radius of a stent flexibly differ following the shape of an outer wall of a luminal structure of a tubular organ in a body so that the stent is in tight contact with the outer wall of the luminal structure. More specifically, regarding the latter, the flexibility in the radial direction, with consideration of not only a stent having lower stiffness, but also the stent being placed in an organ in a body having a tapered structure, it is necessary for a stent to have a property whereby the expansive force of the stent does not change greatly depending on local changes in sectional diameters of the inner cavity at a site having a tapered structure.
The structures of a stent are generally classified into the two types of open cell structures and closed cell structures. Since a stent having an open cell structure exerts remarkable mechanical flexibility in the longitudinal axis direction, the conformability is high and thus the open cell structures have been recognized as being effective for a stent structure that is placed in a tortuous tubular organ. However, for such an open cell structure, since a part of a strut of the stent may protrude radially outward in a flared shape when bent, there is a risk of damaging the tissue of a tubular organ in a body such as blood vessels when the stent is placed therein. On the other hand, regarding stents having a closed cell structure, there are those having closed cell structures that allow for a partial repositioning of a stent during operation, which had been difficult with stents of open cell structures, and stents having closed cell structures that allow for full repositioning of the stent during operation.
For such a closed cell structure, although there is no risk of the strut of the stent protruding radially outward such as a stent having an open cell structure, the flexibility of the structure tends to be lacking. Therefore, there has been a risk of inhibiting the flow of liquid such as blood in tubular organs from flowing due to a stent buckling when applying the stent having a closed cell structure to a bent tubular organ. Furthermore, structurally speaking, since the stent having a closed cell structure is inferior to the stent having an open cell structure in terms of a reduction in diameter, the stent having a closed cell structure cannot handle placement of a stent into a tubular organ of small diameter of around 2 mm, a result of which there has been a risk of damaging a body tissue.
In order to solve such problems, a spiral stent has been devised as a technology exhibiting high flexibility while being a stent having a closed cell structure (for example, refer to Japanese Unexamined Patent Application (Translation of PCT Publication), Publication No. 2010-535075.) The stent disclosed in Japanese Unexamined Patent Application (Translation of PCT Publication), Publication No. 2010-535075 includes spiral circular bodies having a wavy-line pattern and coiled elements connecting adjacent circular bodies in an expanded state.
Patent Document 1: Japanese Unexamined Patent Application (Translation of PCT Publication), Publication No. 2010-535075